What is proposed in the prior-art Patent Documents 1 to 4 are audiphones that conduct audible sound vibrations, not through the narrow and complicated ear canal but through the skull, to the inner ear including the vestibule and cochlea in the skull.    Patent Document 1: Japanese Patent Application No. H9-261797    Patent Document 2: Japanese Patent No. 3174324    Patent Document 3: Japanese Laid-open Patent Publication No. 2004-289219    Patent Document 4: Japanese Laid-open Patent Publication No. 2007-184722
In contrast to the above, among audiphones of such a kind that have been put to practical use, first what is proposed is the one whose audible-sound transducer is strongly pushed by a head strap or the like against the skull through the scalp in order to conduct sound vibrations to the skull from outside the scalp. However, the problem is that a user wearing the audiphone bears a considerable burden.
Secondly, there is an audiphone having a structure in which a titanium bone-conduction terminal is embedded in a postauricular region with an external transducer attached to the titanium bone-conduction terminal. However, the audiphone is inconvenient for use because the titanium bone-conduction terminal is exposed on the skin in the postauricular region. Moreover, the output power of the external transducer is not sufficient; a large value of bone-conduction hearing ability, at least about 45 dB, is required. Therefore, the problem is that the audiphone has limited uses.